Harlequins do eat sertpent stars, but IME serpent stars don't last as long as bulkier stars, not sure if they just can't handle the stress of being eaten of if there is so little flesh that they don't last as long. Brittle and serpent stars do have tube feet, all echinoderms including cucs and sea apples do, it is part of what makes a star a star. As for cost of food, a chocolate chip, which costs about 8 dollars here, lasts about a week, the food for my reef costs more then that per week, and my fish eat about a dollar a day as well, so even though it is only 1 animal, the cost isn't that bad. Also, if you don't let the star rot you don't have too much trouble with water quality, but it can distract from the display tank to have a half eaten star on the sand in full view. IMO this is worth the beauty of the shrimp, and to me, the stars are food, just like any other live food item I introduce.